1. Field of the Invention
The present invention relates to a floating-zone melting process (to be referred to as an FZ process hereinafter), and more particularly, to an improvement of a single-turn induction heating coil used for an apparatus for manufacturing single-crystal semiconductors such as silicon, germanium and gallium phosphide.
2. Description of the Prior Art
In a conventional apparatus, for example, a rod-like polycrystalline semiconductor is held at an upper shaft and a single-crystal seed is held at a lower shaft, the lower end of the polycrystalline semiconductor is melted using an RF induction heating coil and nucleated by contacting with the top of the single-crystal seed, and then the coil and the polycrystalline semiconductor are relatively rotated and relatively moved in an axial direction while a dislocation-free crystal is prepared letting a molten zone to traverse the length of the polycrystalline semiconductor, thereby manufacturing the rod-like single-crystal semiconductor. A single-turn flat heating coil or a multiple-turn flat heating coil is used as an induction heating coil for the apparatus of this type. The single-turn heating coil is advantageous over the multiple-turn heating coil in that zone melting can be performed within a narrower region, and a large current can be obtained at a low voltage. Accordingly, the single-turn flat heating coil capable of concentratively heating a narrow region is widely used for an apparatus for manufacturing dislocation-free single crystals having a larger diameter.
In such a single-turn flat heating coil, for example, as shown in FIG. 5, an inner periphery 51 of a ring-like coil is provided with a tapered section, and end faces 53 of a coil 50 provided with a power supply portion 55 at its outer peripheral wall 52 extremely come close to each other through a gap 54 so that symmetry of a current circuit in a peripheral direction of the coil 50 is maintained, thereby obtaining a substantially uniform magnetic field (Japanese Patent Publication No. 51-24964 and the like, to be referred to as a first prior art)
However, according to the first prior art, since the slit of the coil 50 is formed along a plane perpendicular to the peripheral direction of the coil 50, even if the end faces 53 of the both ends of the coil 50 extremely come close to each other, a nonuniform magnetic field is formed thereat. In addition, since the currents flow in reverse directions to each other radially near the both end faces 53 of the coil at the gap 54, an electromagnetic field along the gap 54 is multiplied by the both currents, thereby undesirably increasing the intensity of the nonuniform magnetic field.
When relative rotation/movement of the semiconductor rod and the heating coil 50 is performed in the nonuniform magnetic field, high-impurity zones and low-impurity zones are repeatedly formed (striation) by a local temperature difference produced by the nonuniform magnetic field in each growth cycle per revolution. If a device is manufactured by using a wafer having such striation, microscopic resistance variations generated at a portion having the striation results in product defects.
In general, the single-crystal semiconductor is manufactured by the FZ process in a protective gas atmosphere using an argon gas or the like. Therefore, as shown in FIG. 6, turbulence of the protective gas occurs near a melting zone 2 of a semiconductor rod 1, and generally, a swift gas flow 3 occurs through the gap 54 along the axial direction of the semiconductor 1, so that the swift gas flow 3 tends to directly collide with a shoulder portion of a polycrystalline semiconductor 4. As a result, a collided portion is locally cooled to remain unmelted and then to grow to a spiky protrusion.
This tendency of forming such a unmelted portion is accelerated by variations in heat conduction through grain boundaries in a polycrystalline semiconductor. Furthermore, once a protrusion is formed on the outer surface, it is more easily cooled than the other part, and the protrusion is left in an icicle like shape with the lapse of time, thereby forming a so called overhang 5.
Accordingly, if the overhang 5 is produced, when the coil 50 is to be relatively moved to the polycrystalline semiconductor, the heating coil 50 cannot pass the overhang 5 because the coil 50 is physically blocked therewith. Furthermore, when the coil 50 comes close to the overhang 5, an undesirable discharge phenomenon is caused therebetween, and hence manufacturing must be stopped at that position.
In order to eliminate such drawbacks, for example, a single-turn induction heating coil such as shown in FIG. 7 is proposed in Japanese Unexamined Patent Publication (Kokai) No. 52-29644 (to be referred as to a second prior art hereinafter).
The arrangement of the second prior art will be briefly described below. In the second prior art, power supply portions 61 and 62 are formed on a coil 60 in the same manner as in the first prior art, wherein the power supply portions extend from an inner peripheral side 60a to an outer peripheral side 60b of both ends of the coil 60. However, the difference is that in the second prior art, the power supply portions 61 and 62 extend from the inner peripheral side 60a to the outer peripheral side 60b while vertically overlapping each other.
In the second prior art, however, since the power supply portions 61 and 62 are vertically overlapped, contrary to the first prior art, electromagnetic fields at the supply power portions 61 and 62 cancel each other or are attenuated from each other, so that the nonuniformity of magnetic field in the peripheral direction of the coil is not solved at all.
When the heating coil 60 is used for manufacturing a single-crystal semiconductor, in order to apply intensified magnetic field to a narrower melting zone, the inner peripheral side 60a need be thinned or wedge-shaped. However, in the second prior art, since the power supply portions 61 and 62 vertically overlap each other, and moreover, extend in the radial direction through a gap 63 between the opposite ends, it is very difficult to form the inner peripheral side 60a to be thinned or wedge-shaped. Consequently, the magnetic concentration cannot be improved.
In addition, since the power supply portions 61 and 62 function as a tube for guiding a coolant in and out, they must be formed into a hollow tubes while each having a sufficient wall thickness against a pressure of the coolant, and moreover, the gap 63 between the vertically positioned power supply portions 61 and 62 need be designed enough to prevent a discharge. Therefore, in any case, the thickness of the coil 60 in the second prior art inevitably becomes considerably large, and hence it is not suitable for an induction heating coil for floating-zone melting of single-crystal semiconductor.